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I have a 2002 330xi. I do not get heat in through the vents unless I disconnect the wire from the heater control valve located in the engine compartment.

With the wire connected to the heat control valve, the heat control motor is extremely hot to the touch and the two heater hoses are cold. Once the wire is disconnected, I get all the heat in the world.

I have changed the climate control panel and still the same problem. The A/C works great and all the other climate control functions work as they should.

On the HVAC control panel, find the line at the rear of the unit with the controlling signal that operates the heat exchanger solenoid vavle. This means removing the unit from the dash and likely disassembling parts of the dash. Once you've found the "control wire" or signal wire that controls the motor to the solenoid operated valve, hook a voltmeter to it and watch the temperature reading display as you change the temperature up and down on the temp up & down buttons to see iff the voltmeter voltage changes as you raise and lower the temp. If the control line gives a dead signal, there's your problem; maybe go to a wrecking yard and buy another one (HVAC Control Unit) or buy a new one. If you are handy enough to disassemble the potentiometer, perhaps you can just replace that part if there are manufacturer part numbers on it. Maybe there's just a broken solder connection that needs resoldering or something simple of that nature.

See of you can obtain wiring diagrams off the Internet also to make your hunt easier to find the signal wire.

It appears you have tested the solenoid operated heating valve and if it is running satisfactory then it might be OK to keep, but since you have the whole dash apart, I would replace the heat exchanger valve, the controller and other bits prone to wear and tear.

I identified the wire on the back of the climatge control panel and checked it with a volt meter. The voltage does not change. I have a constant 12v give or take. I also checked the wire with an ohm meter to make sure that I had the right wire.

After identifying the wire and checking the voltage, I bought a used control panel. Still the same problem, no heat, and still the same voltage. 12v no matter where the temp setting was set at.

To be sure I did not have a second control panel that was bad, I went to the auto recycler and tried a third panel. Same problem - 12v from the climate control panel to the heat control valve at all times. The signal does not change.

I have checked the wiring to the heat control valve. I am getting 12v from the fuse.

With the wires plugged into the heat control valve I get no heat. Once I unplug that wire, the valve opens and I get plenty of heat.

I had the same thing last week, and by searching that it could be the heater core, or it can be as simple as bleeding the cooling system.. Make sure the dial is also on the red setting. I bled the system and Now i'm getting heat again..

Thanks for the info - I have plenty pf heat when I unplug the wiring from the heater control valve motor. My problem is more electircal. Thia one has me stumped. Something is not sending the proper voltage signal to the heater control valve motor.

You are not alone. I have the exact same issue on my '01 325i. A constant 12V to the HCV even when the key is not in the ignition.

I have no answer at this point. I have read on this or another E46 forum that the HCV actually pulses open and closed to control the flow of hot coolant into the heater core. according to the post there are 5 or 6 sensors that control this pulsing.

I just finished replacing the idler belt pulleys, so this is next on my list of things to fix.

There are two sensors located in the heater box that adjust the voltage signal through the climate control panel and into the heater control valve. One of the sensors was defective and it was sending a signal that the temp in the heater box was high heat at all times.

There are two sensors located in the heater box that adjust the voltage signal through the climate control panel and into the heater control valve. One of the sensors was defective and it was sending a signal that the temp in the heater box was high heat at all times.

I had the sensor changed and the problem was solved.

I appreciate your reply. I have been searching for the details of those sensors. Do you have any part number or realoem diagram that they are on perchance?

edit: Looks like I am looking for the "heater core temperature sensor" which seems to be located behind the heater control panel. Does this sound like the sensor you ended up replacing?